1. Field of the Invention
This invention relates to a two-stroke internal combustion engine having a cylinder containing a stepped piston, the cylinder and piston having a pumping part, of larger diameter and a working part, of smaller diameter, there being associated with the cylinder at least one receiver volume arranged to receive, through transfer port means in the working part of the cylinder, a charge from the pumping part of the cylinder and to deliver through said transfer port means the charge to the working part of the cylinder. In such an engine, the piston is provided with recess means which co-operate with the transfer port means to allow and control entry of the charge into the or each receiver volume, the piston also co-operating with the transfer port means to allow and control entry of charge from the or each receiver volume to the working part of the cylinder. Such an engine will hereafter be referred to as an engine of the kind specified. One example of such an engine is described in British Patent Specification No. 1,171,767.
2. Description of the Prior Art
In two stroke engines it is important that the flow of fresh charge into the cylinder is directed away from the exhaust port means in order to reduce the loss of fresh charge by short-circuiting into the exhaust. Loop scavenging (Schnurle System) is the system adopted by virtually all modern engines in order to achieve good scavenging. In this system the transfer port means are symmetrically disposed relative to the exhaust port means and are directed towards the cylinder wall opposite the exhaust port means. The transfer port may enter the cylinder more or less tangential to the cylinder bore. Thus the port means are inclined at an angle to the centre line passing through the exhaust port means. This angle is normally 45.degree. to 70.degree.. It is impossible to manufacture such port means in a casting without using separate cores made in sand or soluble material and individually inserted into the mould or die before the metal is poured. This process does not lend itself to high rates of production in die-casting machines and results in two-stroke engines having a higher cost of production than modern side-valve four-stroke engines using pressure die-cast cylinder blocks.
Existing stepped piston engines of the type described in U.S. Pat. No. 3,550,569, although requiring less component parts than a side-valve four-stroke engine, involve loop scavenge type transfer port means as described above. This has hitherto prevented the stepped piston engine from achieving absolute minimum production costs by the use of die-cast cylinder blocks.